Support password authentication.

Support password authentication.

Add alternative authentication middleware to authenticate against Keystone using
username and password instead of validating existing auth token. The benefit
being that you no longer require admin/service token to authenticate users.

To use, update heat-api.conf:
     [paste_deploy]
     flavor = standalone

This should make it possible to try heat out against existing public
clouds.

Change-Id: Ia584bba78e8984581f0fb6882bbb17d5efa238db